Preheat oven to 425°. Toss apple with sugar in a small bowl.
Meanwhile, melt 3 Tbsp. butter in a medium-size nonstick skillet over medium-high heat; add Vidalia onion, and sauté 6 to 8 minutes or until onion is tender and begins to caramelize.
Combine baking mix and cheese in a large bowl; make a well in center of mixture. Whisk together milk and eggs; add to cheese mixture, stirring just until moistened. Stir in onion and apple. Spoon into a lightly greased muffin pan, filling almost completely full.
Bake at 425° for 14 to 16 minutes or until golden. Let cool in pan on a wire rack 2 minutes. Remove from pan to wire rack, and let cool 6 to 8 minutes. Serve warm.